This invention relates to a network comprising a plurality of station units connected by a ring-like data transmission line enabling circular transmission of communication data in digital form and adopting a master slave method as a communication method, and in particular to an abnormality monitor method and an abnormality monitor system in a network capable of maintaining a network management function in a master station unit in the normal state even if a communication function in the network is impaired because of a broken line of a data transmission line, an abnormality in communication means for governing reception and transmission of communication data, or the like.
Hitherto, a network generally has been known which comprises a plurality of station units connected by a ring-like data transmission line enabling circular transmission of communication data in digital form.
To exchange communication data between station units in a network configured as described above, first a communication IC for governing reception and transmission of communication data is provided for each station unit and the communication the transmitting party in a predetermined direction on a data transmission line. The communication IC in the station unit downstream from the transmitting party on the data transmission line determines whether or not an address of its own station unit is specified as a destination. If the address of its own station unit is not specified as the destination, the communication IC sends the communication data to the station unit downstream from its own station unit through the data transmission line. On the other hand, if the address of its own station unit is specified as the destination, the communication IC reads the communication data through the data transmission line, thereby exchanging the communication data between the station units.
Of the networks configured as described above, hitherto a network adopting a master slave method as a communication method has also been known. This network consists of one master station unit operating as a master and other slave station units operating as a slave.
In the network adopting the master slave method, the master station unit performs operation management of all station units making up the network and maintains the network communication function in the normal state as a network maintaining function, and each slave station unit usually stands by in a reception wait state and upon reception of communication data containing control information for each slave station unit such as transmission start or reception start sent from the master station unit, the slave station unit suitably receives or transmits communication data including source data containing voice or video information and control data containing slave station unit control information.
However, according to the conventional network adopting the master slave method as the communication system, if a broken line of the data transmission line occurs or an abnormal condition such as a failure occurs in a communication IC in a station unit at a midpoint position on the route between the transmitting party and the destination, the communication data cannot be transmitted to station units downstream from the broken data transmission line or the station unit falling into the abnormal condition, thus the master station unit cannot keep track of the abnormality occurrence point in the network. Resultantly, a problem of interfering with the network management function of the master station unit, which should be solved, is contained in the prior art.